1. Field of the Invention
The present invention relates to the driving of an optical modulator and, more particularly, to the driving of an optical modulator to measure, and compensator for, dispersion in an optical transmission line.
2. Description of the Related Art
Optical transmission systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. For example, optical transmission systems at 10 Gb/s are now in practical use. However, as users require larger amounts of information to be rapidly transmitted, and as more users are connected to the systems, a further increase in the capacity of optical transmission systems is required.
In an optical transmission system having a transmission speed equal to or higher than 10 Gb/s, the transmission wavelength is deteriorated by wavelength dispersion so that it becomes difficult to continue transmitting data. As a result, dispersion compensation must be performed.
For example, if a signal light having a wavelength of 1.55 .mu.m is transmitted through an optical transmission system at a transmission speed equal to or higher than 10 Gb/s, using a 1.3 .mu.m non-dispersion single mode fiber, then the wavelength dispersion increases by 18 ps/nm/km. This increase in wavelength dispersion is relatively high, thereby requiring dispersion compensation to be performed.
In an optical transmission system having a transmission speed equal to or lower than 10 Gb/s, the range of dispersion tolerance is relatively large, and dispersion compensation can be performed by commonly applying a dispersion compensator having a predetermined dispersion value. For example, when a signal light of 1.55 .mu.m in wavelength is transmitted through a 1.3 .mu.m non-dispersion single mode fiber (SMF), the dispersion tolerance is about 800 ps/nm. Therefore, a system can be designed in such a way that a dispersion compensator having a predetermined dispersion value, such as a dispersion compensation fiber (DCF) or a fiber grating, is commonly applied for short distance (for example, 20 km through 40 km) transmission systems using a 1.3 .mu.m non-dispersion single mode fiber.
On the other hand, in an optical transmission system having a transmission speed equal to or higher than 10 Gb/s, the range of dispersion compensation tolerance is small, thereby requiring dispersion compensation to be performed with high precision. As a result, the change of dispersion in the transmission line must be measured and then dispersion compensation must be optimized.
FIGS. 29(A) and 29(B) show the results of an experiment indicating the relatively small amount of dispersion compensation tolerance at a transmission speed of 40 Gb/s.
More specifically, FIG. 29(A) is a diagram illustrating an optical transmission system, and FIG. 29(B) is a graph illustrating the deterioration of reception sensitivity (power penalty) versus the dispersion compensation ratio of the optical transmission system. Referring now to FIGS. 29(A) and 29(B), a signal is transmitted at 40 Gb/s from a transmission unit 341 through a 1.3 .mu.m non-dispersion single mode fiber 342 for 50 km, and is then received by a receiving unit 344. Wavelength dispersion occurs at 920 ps/nm. Therefore, by adjusting the length of a dispersion compensation fiber 343, the wavelength dispersion of the 1.3 .mu.m non-dispersion single mode fiber 342 can be compensated for.
As indicated by FIG. 29(B), when a power penalty equal to or less than 1 dB is an acceptable transmission condition, the dispersion compensation tolerance is only 30 ps/nm. Therefore, accurate dispersion compensation is required. However, in the existing transmission line using the 1.3 .mu.m non-dispersion single mode fiber 342, the amount of dispersion is not correctly computed at a number of points. Furthermore, since the amount of dispersion changes with time, depending on, for example, the temperature or the stress applied to an optical fiber, the amount of the dispersion compensation should be appropriately set for each of such intermediate points by accurately measuring the amount and change of dispersion.
Recently, 1.55 .mu.m non-dispersion shift fibers (DSF) have been adopted to perform extra-high-speed data transmission. The wavelength dispersion occurring when a signal light having the wavelength of 1.55 .mu.m is transmitted through this fiber is equal to or smaller than .+-.2 ps/nm/km, and the influence of this dispersion is smaller than that with the 1.3 .mu.m non-dispersion single mode fiber.
However, when the transmission speed is equal to or higher than 40 Gb/s, the dispersion compensation tolerance is very small so that dispersion compensation is required even if the 1.55 .mu.m non-dispersion shift fiber is used. As a result, the amount of dispersion compensation should be constantly set to an optimum value to further prevent deterioration in transmission with time, as with the 1.3 .mu.m non-dispersion single mode fiber.
FIGS. 30(A) and 30(B) are diagrams illustrating the conventional measurement of dispersion in an optical transmission system.
More specifically, FIG. 30(A) illustrates a twin-pulse method of measuring the wavelength dispersion. In the twin-pulse method, a group delay time difference is directly measured from the interval of pulses, to measure the wavelength dispersion.
Referring now to FIG. 30(A), a pulse signal output from a pulse generator 351 is provided to laser diodes 354 and 355 through drive units 352 and 353, respectively. Upon receipt of a pulse signal from drive unit 352, laser diode 354 outputs an optical pulse having a wavelength .lambda.1. Similarly, upon receipt of a pulse signal from drive unit 353, laser diode 355 outputs an optical pulse having a wavelength .lambda.2.
The optical pulse having the wavelength .lambda.1 output from laser diode 354 and the optical pulse having the wavelength .lambda.2 output from laser diode 355 are provided to an optical fiber 357 through a half mirror 356, and transmitted to a detector 358 through optical fiber 357.
When the optical pulses having the wavelengths .lambda.1 and .lambda.2 are transmitted through optical fiber 357, detector 358 outputs a detection result to a sampling oscilloscope 359. Sampling oscilloscope 359 compares the arrival time of the pulse signal received from pulse generator 351 through a delay circuit 360 with the arrival time of the optical pulses detected by detector 358. The amount of dispersion is obtained by detecting the delay difference between the two optical pulses after transmission through optical fiber 357.
FIG. 30(B) illustrates a phase method of measuring the wavelength dispersion. According to the phase method, the group delay time difference is not directly measured, but the wavelength dispersion is obtained from the phase difference between the optical modulation signals generated by the group delay time difference.
Referring now to FIG. 30(B), a synthesizer 371 modulates an optical signal output from laser diodes 372 through 374. Laser diode 372 outputs an optical signal having a wavelength .lambda.1, laser diode 373 outputs an optical signal having a wavelength .lambda.2, and laser diode 374 outputs an optical signal having a wavelength .lambda.3. The optical signals output from laser diodes 372 through 374 are switched by an optical switch 375, provided to an optical fiber 376, and transmitted to an avalanche photodiode 377 through optical fiber 376.
Avalanche photodiode 377 converts the optical signal transmitted through optical fiber 376 into an electric signal, and outputs the electric signal to a vector voltmeter 379 through an amplifier 378. Vector voltmeter 379 compares the electric signal transmitted from synthesizer 371 with the electric signal transmitted from amplifier 378, and obtains the phase difference between the optical modulation signals, thereby computing the amount of dispersion.
However, a relatively large number of parts, such as laser diodes, are required by a conventional optical transmission system when the dispersion measurement systems shown in FIGS. 30(A) or 30(B) are incorporated into a transmitting/receiving unit as a part of the transmission system in order to set the amount of dispersion compensation to an optimum value. Therefore, the entire system becomes large and costly.